In the art of disposable diapers the most practical material for use as the main absorbent pad element from a unit cost standpoint is an air-formed fluff batt of cellulosic wood fibers. The majority of disposable diapers now being marketed employ such elements. Batts made from these fibers have a low density and a high capacity for fluid absorption but have very poor wicking ability. As a result the areas which become wetted by absorbed fluid in an unmodified fluff batt during use are basically controlled by gravity and are relatively small with respect to total area available leaving much potential capacity unused. Generally speaking, the areas of greatest fluid acceptance and therefore the most efficient with respect to utilization of potential absorbent capacity are the areas of an absorbent fluff batt closest to the point of initial wetting, or at the lowest position of the structure in use in relation to gravity flow. In other words, as batt areas become farther removed from the initial point of wetting and/or are located at higher positions in relation to gravity flow from the point of wetting during use they become less efficient with respect to utilization of potential absorbent capacity.
It is well known that the wicking ability of a fluff batt, as well as other types of fibrous batts, is improved by increasing its density through the use of overall compression, line embossing, pattern embossing, or the like whereby pore sizes in the densified batt area are reduced, thereby increasing capillary suction pressure to such extent that absorbed fluids will overcome the force of gravity sufficiently to be capable of moving into the densified areas from the less dense areas. One of the first to recognize this phenomenon was Heitmeyer in U.S. Pat. No. 1,863,333. More recent variations are found in Morin 2,788,003; Burgeni 2,952,259-60; Gobbo Sr. et al 3,065,751; Bletzinger et al 3,375,827; Murphy 3,430,629; Krusko 3,766,922; and DeNight et al 3,769,978. As indicated in these patents, it is also known that while capillarity is improved by densification which reduces average pore size, the total fluid receiving capacity of a fluff batt is decreased as the average pore size is decreased. Thus, while the use of embossing or other means of densification does obtain better distribution of fluid within a fluff or other fibrous batt it does so with some loss in capacity and is at best a compromise, i.e., a trade-off of better distribution of fluid throughout the available area for a decrease in total capacity for the same area. In most cases the advantages of using more of the available area tends to overshadow slightly the loss of capacity and is usually the design of choice. In instances such as in U.S. Pat. No. 3,769,978, where line-embossing is utilized to improve distribution, larger quantities of fluff are placed in the area where initial wetting takes place and where maximum capacity is most needed in order to make up for loss in capacity which results from the embossing. While this construction improves fluid distribution it does so at an added material cost which is undesirable from an economic standpoint.
Applicant has now found how to structure a diaper to achieve distribution of absorbed fluid into those areas of the diaper where distribution has been lacking or poor during use, and thereby effectively utilize most of the available capacity in those areas, without at the same time reducing potential capacity where distribution is already optimum during use and without requiring the use of additional absorbent material.
The present invention is therefore directed to a structure designed to use the fluid absorbent capacity of a fluff batt in those areas which because of poor flow characteristics were hitherto substantially inaccessible to fluids and therefore unused, such areas being in a location farthest removed from the initial wetting point and/or in a location highest in relation to gravity flow from the wetting point. In the structure disclosed, there is no impairment or reduction in the absorbent capacity of the more efficient areas of the diaper which are located at or near the initial point of wetting and which are positioned lowest in relation to gravity flow from that initial point of wetting. This improved capability is accomplished by retaining that part of the batt which has a high efficiency with respect to absorption properties in its relatively undensified condition while selectively densifying only that portion of the batt which, when not densified as defined herein, is generally not wetted in use. This structure increases capillary pressures in the hitherto unused areas to an extent that fluid will be drawn out of the less dense area even against the influence of gravity.